This invention relates to power cables in general and more particularly to an improved power cable having plastic insulation and an outer conducting layer.
In power cables for use with transmission voltages of 10 kv or more, an electrically conducting or electrically slightly conducting layer is generally arranged between the conductor and the insulation along with such another such layer being arranged between the insulation and the metallic shield. The purpose of these layers is for controlling the electric field in the insulation. In particular, such layers are used in cables having an insulation made from a polyethylene base in which polyethylene or copolymerisates of ethylene may be present in the thermoplastic or in the interlinked state. In particular with cables having extruded plastic insulation the inner and outer conducting layers are applied using an extrusion process, in an attempt to obtain welding together of the conducting layers and the insulation. Generally, such is accomplished through application of the conducting layers and insulation in the same operation, for example through the use of extrusion heads arranged in tandem or through the use of a triple extrusion head.
When installing fittings in power cables having plastic insulation and having an outer conducting layer provided on top of the insulation it is important that the outer conducting layer be easily removable. In power cables using insulation of interlinked polyethylene, for example, a difficulty has existed in prior art cables in that the outer conducting layer which is applied over the insulation either adheres too weakly to the insulation and has a negative effect on the dielectric strength of the cable or is bonded too tightly to the insulation leading to difficulties in the installation of the cable, i.e. when removal of the outer conducting layer is necessary.
In view of these difficulties the need for an improved cable construction in which dielectric strength is maintained but in which the outer conducting layer can be easily and reliably removed becomes evident.